Purple loosestrife (Lythrum salicaria L., Lythraceae), a Eurasian perennial wetland plant, was introduced into North America in the early 1800's. Negative impacts on biodiversity of infested wetlands and the inability of chemical, physical or mechanical means to provide long-term control resulted in the importation and release of host-specific phytophagous insects from the native range of the plant, a classical biological weed control program. Of nine host-specific insects species considered as control agents, three (the two leaf feeders Galerucella calmariensis L., and G. pusilla Duftschmidt [Coleoptera: Chrysomelidae] and a root feeder Hylobius transversovittatus Goeze [Coleoptera: Curculionidae]) were introduced into North America in 1992. An additional species that attacks the flowers (Nanophyes marmoratus Goeze [Coleoptera: Curculionidae]) was introduced in 1994.
Because as much as 60% of the annual biomass production of L. salicaria is stored below-ground and rootstocks of over 1 kg fresh biomass are common in mature plants, the root-feeding weevil Hylobius transversovittatus is considered a very important one of the host-specific insect species introduced and a prime candidate as a biological control agent. Attack by H. transversovittatus reduces flowering period, plant height, and plant biomass and changes biomass allocation patterns resulting in reduced performance of purple loosestrife.
It has been rather difficult to establish viable field populations of H. transversovittatus. The low abundance of this species in Europe prevented field collections for release in North America. Adults for initial introductions in 1992 were offspring of weevils collected across Europe. Larvae were reared on potted plants (of purple loosestrife) in a common garden in Germany. A total of 1,331 adults and 30,830 eggs were released in nine states and two Canadian provinces through 1993 and initial establishment was confirmed. Rearing of H. transversovittatus has continued at Cornell University and to a lesser extent at facilities of the Bureau of Reclamation in Denver, since 1993. This has allowed the distribution of eggs for field release despite the low number of adults. Although well over 100,000 eggs were distributed by 1999, field establishment rates appear rather low.
Increasing the availability of adults for field release has been considered crucial to increase field populations of the weevil to avoid negative effects due to inbreeding in small populations which could jeopardize the biological control program targeting L. salicaria.